152 Electrical Power Systems Technology
phase transmission line, and three-phase power is more economical than
single-phase power to distribute from plants to consumers that are located
a considerable distance away. Fewer conductors are required to distribute
the three-phase voltage. Also, the equipment that uses three-phase power
is physically smaller in size than similar single-phase equipment.
HIGH-SPEED AND
LOW-SPEED GENERATORS
Generators can also be classified as either high-speed or low-speed
types (Figure 6-12). The type of generator used depends upon the prime
mover used to rotate the generator. High-speed generators are usually driv-
en by steam turbines. The high-speed generator is smaller in diameter and
longer than a low-speed generator. The high-speed generator ordinarily
has two stator poles per phase; thus, it will rotate at 3600 rpm to produce
a 60-hertz frequency.
Low-speed generators are larger in diameter and not as long as high-
speed machines. Typical low-speed generators are used at hydroelectric
power plants. They have large-diameter revolving fields that use many
poles. The number of stator poles used could, for example, be twelve for
a 600-rpm machine, or eight for a 900-rpm generator. Notice that a much
larger number of poles is required for low-speed generators.
Figure 6-12. High- and low-speed AC generators